When a vehicle coasts, a fuel cut is conventionally performed to shut off the supply of fuel to an internal combustion engine. Performing a fuel cut makes it possible to reduce extra fuel consumption. However, when a fuel cut is performed, the torque (indicated torque) output from the internal combustion engine exhibits a stepwise decrease to zero. Such a stepwise torque change may cause a shock depending on the magnitude of an output torque generated immediately before the fuel cut.
A method for alleviating a shock caused by a fuel cut is described in JP-A-1996-246938. This method first reduces the output torque by retarding the ignition timing before shutting off the supply of fuel, and then shuts off the supply of fuel. Further, this method retards the ignition timing until the resulting retardation amount reaches a critical retardation amount.
The aforementioned critical retardation amount is the limit of a retardation amount range where the combustion in an internal combustion engine is maintainable. If a misfire is to be ignored, the ignition timing can be retarded beyond such a limit. When the ignition timing is to be retarded while ignoring a misfire, the torque can be reduced to the minimum torque that can be output from the internal combustion engine. However, the conventional technology described above cannot retard the ignition timing beyond the critical retardation amount because it performs a guarding function for the ignition timing retardation amount. In other words, the output torque obtained immediately before a fuel cut is equal to the torque provided by the critical retardation amount.
The torque provided by the critical retardation amount is higher than the internal combustion engine's minimum torque, which is provided irrespective of misfiring. Accordingly, a great torque change occurs when a fuel cut is performed during the use of the torque provided by the critical retardation amount. To further suppress the shock by reducing such a torque change, it is necessary to permit ignition timing retardation beyond the critical retardation amount and reduce the output torque prevailing immediately before a fuel cut to the internal combustion engine's minimum torque. However, if the ignition timing is merely left unguarded, proper combustion cannot be assured during a normal operation. This will cause another problem such as the generation of a misfire-induced torque shock.
The above problem concerning an abrupt torque change brought about by a fuel cut also applies to the recovery from a fuel cut. When recovery from a fuel cut is achieved, the internal combustion engine generates an output torque stepwise. To suppress the shock by reducing the torque change, it is preferred that the output torque generated upon recovery be minimized. However, if the ignition timing retardation amount is guarded, the output torque generated upon recovery is equal to the torque provided by the critical retardation amount. It means that a torque lower than provided by the critical retardation amount cannot be generated.